Reflection channel incorrect? Or is it me?

[GeneCrucean]

So I've never been one to be all anal about passes perfectly matching a full beauty pass even though I probably should be. However lately I've been trying to level up a few of the skills I consider to be less developed. Enter, comping from a compers pov and not a 3D artists pov. When I was at *a certain studio*, one of my good buddies (comper) made a comment that the 3d department sucked (forgetting that I was in THAT department :) ), I knew he was just frustrated at something but I kind of get where he's coming from because I comp from time to time, not to mention constantly spit out passes for those guys. You get passes that don't match or aren't setup properly or insert a million things that can happen to passes in production here. It's easy to do.

Of all the problems I can encounter, I run into a really simple one right out of the box. Reflections.

I have a very simple scene setup (http://www.genecrucean.com/TEMP/CompingTest.rar, 6.5 scene + fusion file) but maybe all that Sex, Drugs and Rock and Roll (yeah right) finally got to me. Either way, I can NOT get this simple reflection pass to match the Beauty_Main. I'm starting to think it's a problem with XSI's reflection channel but hopefully it's just something stupid I'm doing in fusion.

Here is a quick video (techsmith codec) that shows what is going on. http://www.genecrucean.com/TEMP/CompingTest.avi

To me this pass should be Screened. What am I doing wrong?

[GeneCrucean]

Sorry guys I effed up the scene when merging to remove a render property.

While fixing the scene real fast I slapped together a quick comp in Illusion to see if it was an issue with Fusion's merge node. It's not.

New file is at same link as above

[GeneCrucean]

..... bump

[CiaranM]

Yeah, there's definitely a discrepancy somewhere...have you tried to output the reflection as a standard pass shader, rather than extracting a channel?

[GeneCrucean]

Well, I'm really just trying to get to the bottom of why this reflection channel doesn't match. If it's a bug I'm going to report it.

Actually I am just going to report it so someone from Soft takes a look.

[3DMastermind]

Hi Gene.
I took a look at the scene.
You are correct, there is a discrepancy there with reflection.
You are 'screening' the reflection but in reality we don't really know the exact formula that MR executes internally.
It could be a conflict with the FG's reflection "trace depth" and the way it gets factored in.
Also, I don't know if the overlay "order" plays a role here, but I don't think so.
Nice scene set-up btw.
I will escalate the issue.

[GeneCrucean]

Interesting. I posted over on lamrug and Bart mentioned that the internal comping is calculated by the vendors and not MR.

It isn't internal to mental ray. It depends on the design of the shader which combines them.

For example, as a component, with the mib_color_mix, you can choose.

The 3D vendors each write their shaders the way they want to deal with this.

Thanks for looking into this btw.

[GeneCrucean]

So... since Screen doesn't make sense to comp a reflection pass with, does it make sense to make the buffer output something that will work with Add? Or maybe just Over with a correct alpha?